Src kinases are activated in a number of human tumors but the mechanism by which they mediate their oncogenic functions remains unkown. We have searched for cell cycle substrates of src family kinases and purified and cloned a novel protein that undergoes tyrosine phosphorylation during the mitotic phase by src and yes. This protein, named Trask (Transmembrane and Associated with Src Kinases), is a 140kd transmembrane glycoprotein seen only in more complex organisms. It has little homology to currently known protein families and no functionally revealing sequence motifs. Tet-inducible overexpression of Trask in MDA-468 breast cancer cells leads to a loss-of-adhesion phenotype and, consistent with a function in cell adhesion, Trask interacts with a number of membrane and extracellular matrix metallo and serine proteases, cadherins, syndecans, and fibronectin. Trask is unique among adhesion molecules in that it is under cell cycle regulation. We hypothesize that the mitotic phosphorylation of Trask regulates the transient loss of adhesion seen in epithelial cells undergoing mitosis, and that hyper-phosphorylation or inappropriate interphase phosphorylation of Trask by activated src kinases in tumors leads to aberrant cell detachment and development of a metastatic phenotype. In this proposal, we seek to test our hypothesis. In aim 1 we will determine whether Trask phosphorylation and interaction with extracellular proteases and adhesion proteins regulates cell adhesion and whether the phosphorylation and protease binding activities of Trask are normally restricted to the mitotic phase in epithelial cells. In aim 2 we will determine whether overexpression of Trask in experimental xenograft tumors increases their metastatic rate, and by studying Trask expression in early and metastatic human tumors we will determine whether this reflects the biology of naturally occurring human cancers. In aim 3, we will determine whether metastases can be averted in the src-driven polyoma mT transgenic model of metastatic breast cancer by inactivation of the Trask gene. Our hypothesis, if confirmed, will profoundly improve our understanding of the molecular basis for tumor metastases and reveal the oncogenic role of src kinases, and lead to therapeutic strategies to prevent this lethal complication of cancer.